The manufacture of semiconductor circuits or chips having electronic circuits formed thereon is primarily accomplished with photolithographic techniques. During this manufacturing process, successive layers of circuit patterns are formed onto a semiconductor wafer by projecting the image of a mask containing the circuit patterns thereon onto a wafer having a photosensitive resist coating. The feature sizes of the circuit elements formed on the semiconductor wafer are typically in the range of 0.50 microns. This extremely small feature size, in combination with the required multiple layer used in forming a semiconductor chip, necessitates the use of very accurate alignment systems to align the wafer and mask.
One such alignment system is disclosed in U.S. Pat. No. 4,697,087 entitled "Reverse Dark Field Alignment System for Scanning Lithographic Aligner" issuing to Frederick Y. Wu on Sep. 29, 1987, which is hereby incorporated by reference. Therein disclosed is an alignment system wherein a wafer having a wafer target thereon and a mask having a mask target thereon are aligned with respect to each other. The alignment system has two optical channels or arms used to detect alignment targets in scribe alleys above and below the mask pattern being imaged on the semiconductor wafer. A portion of the light path used in the two optical channels or arms is through the projection optics. This path is not on the optical axis of the projection optics, but is off axis. The optical alignment channels or arms are additionally movable to accommodate different projection field heights. This system is referred to as an off axis through the lens alignment system. While an off axis alignment system has several advantages, it is complex and requires correction due to off axis lateral color aberrations in the projection optics. This adjustment for off axis color aberrations must be periodically recalibrated and adjusted for different optical alignment channel or arm positions.
FIG. 1 is a simplified schematic illustration of the alignment system disclosed in U.S. Pat. No. 4,697,087. A wafer 10 is movable in both X-Y directions by wafer stage 12. Wafer stage 12 has adjacent and attached thereto an autocalibration detector 14. A mask 20 is attached to a mask stage 22. The mask stage 22 typically moves along a single axis in a plane parallel to that of the movement of wafer stage 12. This is typically in the Y direction. Between the wafer 10 and mask 20 is positioned projection optics 16. The projection optics 16 image the mask 20 onto portions of the wafer 10. The illumination system for imaging the mask 20 onto the wafer 10 is not illustrated and does not form a part of this invention.
Also between the mask 20 and the wafer 10 is a beamsplitter 18. Associated with the beamsplitter 18 are two alignment channels. One channel is formed by first channel alignment optics 26, first channel illumination source 28, and wafer target detector 29. The second alignment channel is formed by second channel alignment optics 30, second channel alignment illumination source 32, and wafer target detector 33. The first alignment channel has an optical axis 34 and the second alignment channel has an optical axis 36. Both the alignment channels are off the axis of the projection optics. The optical axis of the projection optics is illustrated by line 38. The alignment channel illumination paths follow a special path through the projection optics corrected at visible alignment wavelengths. Each alignment channel in combination with mask target detector 24 is capable of detecting the location of both wafer and mask alignment targets while the wafer and mask are simultaneously scanned. During a single alignment scan, the relative position between a series of wafer and mask targets located in scribe alleys at both the top and bottom of a field are measured. Multiple wavelengths of the alignment illumination must be optically separated with each color collected on a separate photo detector. The alignment shifts due to off-axis lateral color aberrations are corrected after alignment data is collected. The visible light path of the alignment illumination must be corrected over the full field height. Because the two optical alignment channels are movable to accommodate different field heights, an autocalibration detector 14 must be mounted on the wafer stage and is used to periodically measure the offsets between the visible light alignment channel paths and the actinic exposure projection optics path for different channel positions, and alignment wafer lengths. Mask target detector 24 is used to detect the targets on the mask 20, and may be used in combination with wafer target detectors 29 and 33. The operation of this off-axis through the lens alignment system is controlled by control means 39.
FIG. 2 illustrates another prior art alignment system. This optical alignment system is very similar to that disclosed in FIG. 1, with the exception that a single channel non-through the lens optical alignment system is used. The non-through the lens alignment system in FIG. 2 is illustrated by alignment illumination source 40, wafer target detector 41, and alignment optics 42. The single channel alignment system has an alignment optical axis 44. The single channel optical alignment system is mounted adjacent the projection optics 16. The position of wafer targets on wafer 10 is measured when the wafer is moved in position in the field of the alignment system and scanned by movement of wafer stage 12. The position of the mask alignment targets is measured by autocalibration detector or actinic light detector 14 when moved in position by wafer stage 12. Therefore, detection of both wafer and mask targets during a single alignment scan is not possible, and the relative positions of the wafer target detector 41 and the projection optics 16 image must remain stable between sequential measurements. This single off-axis viewpoint also requires that the wafer stage 12 have a high accuracy over a large range of travel and that optical magnification and systems skew be separately measured. The operation of this non-through the lens alignment system is controlled by control means 43.
While these and other prior alignment systems are adequate for their intended purposes, there is an ever increasing need for a simpler, more reliable, and more efficient optical alignment system for aligning a wafer and mask during manufacture of semiconductor chips.